The present invention relates to a hand-held power tool and, more particularly, to a power screwdriver having a user selectable shaft locking arrangement to permit the operation of the tool in a manual non-powered mode whereby greater torque can be delivered to a screw.
It is well known that the torque or force which must be applied to a screw can vary substantially as the screw is advanced into, or removed from, anchoring engagement with the associated surface. In particular, very large forces may be required to ultimately `set` the screw during installation or to initially `break loose` the screw during removal. Difficulties have been encountered in generating these large driving forces, particularly in small hand-held tools where size and weight considerations necessarily limit the available motor horsepower. The problem is further aggravated where cordless battery tool operation is contemplated. In such a tool it has been found that batteries of sufficient electrical capacity to power a high-torque cordless screwdriver tool are impractical due to inherent size and weight limitations. This is particularly true with respect to the cordless screwdriver of the instant invention which advantageously features an exceptionally compact, lightweight and completely self-contained construction particularly suited for use in a variety of difficult to access as well as conventional environments.
Proposed solutions to the low torque problem have been found to be objectionable. One such solution provides an increased gear reduction from the motor to the drive bit which correspondingly increasing available torque. This arrangement, however, lowers the drive speed thereby defeating one of the principal advantages of powered operation. Alternatively, a conventional screwdriver may be utilized where necessary to deliver high torque in conjunction with a battery-operated screwdriver of reasonable shaft speed. However, the necessity for frequent switching between these tools renders this solution unattractive.
The compact battery-operated screwdriver of the instant invention advantageously combines, in a single tool, the high speed effortless drive capability of a motor powered screwdriver with the high torque feature of manual screwdrivers often required to set or loosen screws. This combination is achieved by a convenient, user accessible arrangement which rigidly locks the drive shaft and screwdriver bit in fixed relationship to the overall tool housing. In this way, the motor and battery housing serve as a rigid handle for the screwdriver but thereby permitting the manual delivery of substantial torque forces to the screw by the user. It will be appreciated that this arrangement facilitates nearly instantaneous switching between the powered and high torque operating modes without need to withdraw the tool from engagement with the screw or regrasp the tool by the user. The user simply holds the tool by its handle in a conventional manner urging it into engagement with the screw and, as necessary, actuates the locking arrangement of this invention with the other hand.
U.S. Pat. No. 3,789,933 to Jarecki and U.S. Pat. No. 3,809,168 to Fromm, assigned to the present assignee, describe electric power drills having a coupler sleeve arrangement interposed between the drill housing and the bit spindle or chuck. These couplers are adapted to permit the selective operation of the drill in the conventional drill mode, in a hammering mode, or in a combination drill and hammer mode. Neither of these patents teaches means for locking the spindle whereby the power tool may be utilized as a high-torque manual screwdriver.
It is therefore an object of the present invention to provide a compact and lightweight screwdriver tool having conventional power tool shaft speeds generally in excess of those obtainable by manual drive means; the tool may be battery-operated and generally capable of supplying sufficient torque force to advance or remove screws but including a shaft locking arrangement whereby the user may rapidly lock or unlock the shaft in rigid relationship to the housing to facilitate use of the tool either in the powered high speed mode or in a manually operated high torque mode. It is a further object of the invention that the shaft lock arrangement be readily and easily operable and located to permit substantially instantaneous switching between modes by the user while maintaining tool engagement with the screw and normal hand positioning on the tool.